1. Field of the Invention
The present invention is directed generally to measurement systems and more particularly to measurement systems using ultrasonic signals.
2. Description of the Prior Art
The measurement of fluid levels in tanks, pressure vessels, boilers, and the like (hereinafter referred to as vessels) is essential for control purposes, maintaining inventories, and insuring safety. Many techniques are available for measuring fluid levels including sight glasses, float switches, and differential pressure monitors. Acoustic measuring systems, which have gained some acceptance in fields such as photography, have also been adapted for measuring fluid levels in vessels. In currently used systems, acoustic waves are typically propagated through air or other gas using transducers mounted above the liquid surface. An acoustic wave propagating in an extended medium attenuates at a rate which is inversely proportional to the square of the distance from the source of the wave. That attenuation greatly limits the useful measurement range and therefore presents a significant drawback in adapting such technology to the measurement of fluid levels in large vessels.
Often the measurement of the fluid level in a standpipe, which is in fluid communication with the vessel through taps carried by the vessel, can provide an indication of the level of fluid in the vessel. However, a problem arises in that the height of the fluid in the standpipe H.sub.s may be less than the height of the fluid in the vessel H.sub.v. That can arise when heat is being added to the vessel but the standpipe is not maintained at the same temperature due to heat loss to the environment. That difference in temperature between the vessel and the standpipe results in a difference in density between the fluid in the vessel and the fluid in the standpipe. Because the fluid in the standpipe has a different density than does the fluid in the vessel, the level of fluid in the standpipe H.sub.s is not the same as the level of fluid in the vessel H.sub.v.
The difference between the fluid level in a boiler and the fluid level in the boiler's standpipe H.sub.v -H.sub.s is shown in FIG. 1 as a function of the height H between the taps. Three curves are shown which represent three different operating temperatures and pressures. As can be seen from the figure, as the volume of fluid in the boiler increases, and as operating temperatures and pressures increase, the difference between the two levels increases.
To compensate for such level differences, some sight glasses are fitted with thermometers so that operators not only see the fluid level, but they can also read the temperature of the fluid. Based on the temperature reading and the operator's experience, the operator then determines how much higher the vessel's fluid level must be. Clearly, such a system is heavily dependent on the experience level of the operator and costly mistakes can easily be made. Thus, the need exists for a system which can provide a temperature compensated measurement which is indicative of the height of the fluid in a vessel and which is more easily understood by operators.